FLORIDA GEOLOGICAL SURVEY


mgd. which was reached in 1928. After 1928, the chloride content of the
water pumped at Newport should have increased about 3 years later,
or by 1931. The rise in salinity did not become apparent until 1937. The
salt water probably took about 6 years to pass through the 20-foot thick
bed of clay.
 An example of rapid encroachment occurred along the Escambia
River at the Chemstrand nylon plant. The salt-water encroachment into
the sand-and gravel aquifer at the Chemstrand plant comes from the
Escambia River by lateral movement. This conclusion was reached after
considering all the known factors. Prior to pumping at the Chemstrand
plant, ground water moved eastward toward the Escambia River and
seeped into the river. Ground-water pumping lowered the ground-water
level and eventually caused water from the Escambia River to infiltrate
into the ground. The water that infiltrates is salty part of the time.
The river infiltration explains why the ground-water levels at Chemstrand
have generally stabilized.
 The other possible source of salt-water encroachment at the Chem-
strand plant is from salt water below the sand-and-gravel aquifer. A
study of resistivity logs of wells indicates that the water in a thick clay
section (fig. 6) below the sand-and-gravel aquifer at Chemstrand is salty.
As the clay is virtually impermeable, the salt water is not believed to
come from this source. This thick clay bed prevents salt water from
moving up from the limestones below it. In addition, the chloride con-
tent of water from the upper limestone of the Floridan aquifer is only
400 ppm which is not salty enough to cause the high chloride content
sometimes found in well 035-714-4.
 The Bucatunna Clay Member is more than 200 feet thick at the
 Chemstrand plant. The effectiveness of this clay bed as a confining
 layer can be inferred by a comparison of the chloride content of water
 from the adjoining limestone beds. The chloride content of water from
 the upper limestone (above the Bucatunna) is 400 ppm and the chloride
 content of water from the lower limestone (below the Bucatunna) is
 7,3() ppm.
 Well 035-714-4 (Chemstrand Well No. 3) is the production well near-
 est the Escambia River. The water from this well was the first to show an
 increase in the chloride content. If the salt-water encroachment was from
 below, the center wells of a well field are usually the first ones to show
 an increase in the salt content of the water. In 1955, nearby pumping
 caused the ground-water level near the river to decline below sea level
 for the first time and it has remained below sea level most of the time
 since June 1955. In 1956, the chloride content of water from well 085-